Refulgent means ‘to shine very brightly’, hence the antonym of it must be a word that means colorless or less bright, hence option B is the correct answer because “dull” means something that lacks brightness, shine or sheen.
Option A is incorrect because ‘angry’ means feeling or showing strong annoyance, displeasure, or hostility; full of anger, which is not the opposite of brightness hence it is incorrect.
Option C is incorrect because ‘sad’ means being unhappy or upset, hence it is incorrect.
Option D is incorrect because ‘shiny’ means very bright and reflecting, hence it is incorrect as it is not the antonym of refulgent.